This invention relates to a process and plant for separating air.
The most important method commercially for separating air is by rectification. In such a method there are typically performed steps of compressing and purifying the air, fractionating the compressed, purified, air in the higher pressure column of a double rectification column comprising a higher pressure rectification column and a lower pressure rectification column. Condensing, by indirect heat exchange with oxygen-rich fluid separated in the lower pressure column, nitrogen vapour separated in the higher pressure rectification column, employing a first stream of a resulting condensate as reflux in the higher pressure rectification column and a second stream of the resulting condensate as reflux in the lower pressure rectification column, withdrawing an oxygen-enriched liquid air stream from the higher pressure rectification column, and introducing an oxygen-enriched vaporous air stream to the lower pressure rectification column, and separating the oxygen-enriched vaporous air stream therein into oxygen-rich and nitrogen-rich fractions.
The purification of the air is performed so as to remove impurities of relatively low volatility, particularly water vapour and carbon dioxide. If desired, hydrocarbons may also be removed.
At least a part of the oxygen-enriched liquid air which is withdrawn from the higher pressure rectification column is typically completely vaporised so as to form the vaporous oxygen-enriched air stream which is introduced into the lower pressure rectification column.
A local maximum concentration of argon is created at an intermediate level of the lower pressure rectification column beneath the level at which the vaporous oxygen-enriched air stream is introduced. If it is desired to produce an argon product, a stream of argon-enriched oxygen vapour is taken from a vicinity of the lower pressure rectification column below the oxygen-enriched vaporous air inlet where the argon concentration is typically in the range of 5 to 15% by volume and is introduced into a bottom region of a side rectification column in which an argon product is separated therefrom. Reflux of the side column is provided by a condenser at the head of the column. The condenser is cooled by a part or all of the oxygen-enriched liquid air withdrawn from the higher pressure rectification column, the oxygen-enriched liquid air thereby being vaporised. Such a process is, for example, illustrated in EP-A-377 117.
The deployment of a side rectification column to separate an argon product from the air tends to add to the thermodynamic inefficiency of the lower pressure rectification column. Not only does this added inefficiency tend to increase the overall power consumption of the process, it may also cause there to be a reduction in the recovery (i.e. yield) of one or both of the argon and oxygen products in certain circumstances. These circumstances include those in which the rectification columns are required to separate a second liquid feed air stream in addition to the first vaporous feed air stream. Such a second liquid air stream is required when an oxygen product is withdrawn from the lower pressure rectification column in liquid state, is pressurised, and is vaporised by heat exchange with incoming air so as to form an elevated pressure oxygen product in gaseous state. A liquid air feed is also typically employed in the event that one or both of the oxygen and nitrogen products of the lower pressure rectification column are taken in liquid state.
It is an aim of the present invention to provide a method and plant that enable the aforesaid problems, or at least one of them, to be ameliorated.